Gas-liquid contacting towers or columns referred to, for example, as fractionation, distillation or aborption towers are well known particularly in the petroleum and petrochemical industries.
Such towers or columns are designed to conduct liquids in a zig-zag course downwardly through the column while admitting gases upwardly into horizontal-flowing portions of the liquid for intimate contact with the liquid.
Trays for providing the horizontal flow of the liquid are well known in the art and have been widely used. Such trays generally comprise a perforate gas-liquid contacting member or members for effecting intimate contact between a rising gas and a liquid flowing on the surface of the tray across the perforate member. The perforate gas-liquid contacting member is in some instances provided with bubble caps or valves. Adjacent one edge of the contacting member of the tray is an imperforate liquid inlet area for receiving the liquid onto the tray. Adjacent an opposite edge of the contacting member is the liquid discharge end or region of the tray provided with an imperforate weir member extending vertically above the surface of the tray. The flowing liquid overflows the weir member for discharge from the tray. Accordingly, the imperforate weir member, which is often referred to in the art as an outlet weir, maintains a given liquid depth or froth height on the tray.
Those skilled in the art are well aware of the problems associated with designing a tray which will operate in a stable condition over a fairly broad turn-down ratio, that is at a reduced feed rate and therefore at reduced internal liquid and gas flow rates. Tray instability is evidenced by what is referred to as "weeping" and in the extreme as "dumping" which result in decreased tray efficiency, since the desired degree of intimate gas-liquid contacting does not occur due to a lack of liquid inventory on the tray. Weeping is the passage of some of the liquid through the perforations or gas passageways of the gas-liquid contacting member of the tray. Dumping is the condition in which substantially all of the liquid falls through the gas passageways of the gas-liquid contacting member rather than flowing across the tray.
Weeping and dumping are associated with reduced gas flow velocities at turn-down. When weeping or dumping occur, the gas that is prevented from passing through the gas passageways in those sections of the gas-liquid contacting member of the tray must pass through the gas passageways of other sections of the gas-liquid contacting member. This increases superficial gas velocities in these other sections and results in nonuniformity of gas-liquid contact which also adversely effects overall tray performance.
The prior art has employed various approaches to provide for stable tray operation over fairly broad conditions of turn-down.
One approach has been to design trays with excessive tray stabilities by providing a higher than necessary dry plate pressure drop at the design gas flow. Dry plate pressure drop may be simply defined as the pressure drop caused by gas flow through the gas passageways absent the effect of the supported liquid flowing across the tray. As is known in the art, tray stability may be mathematically expressed as being proportional to the ratio of dry plate pressure drop to the height of clear liquid flowing across the tray.
As a stable column is turned-down (i.e., as the liquid and gas flows through the column are reduced) the main resistance to liquid flow through the gas passageways (gas flow) is decreased. The magnitude of this resistance is proportional to and reflected by the magnitude of the dry plate pressure drop. The size of the gas passageways and the surface tension of the liquid are also important factors, but they may be ignored for purposes of understanding this prior art design approach. When turn-down occurs, the main force contributing to liquid flow down through the gas passageways is only slightly effected. The magnitude of this force is proportional to and reflected by the clear liquid head or depth on the tray. In practice, the liquid is supported on the tray as a highly aerated froth. Since this froth has a density lower than an equivalent height of clear liquid (i.e., the process liquid without significant admixture with the process gas) an appropriate correction must be made to transform froth height to clear liquid height. The liquid or froth depth is primarily fixed by the height of the tray imperforate outlet weir at the tray discharge end. Thus, when turn-down takes place, the liquid or froth depth on a tray tends to remain fairly constant. As a result, as the column is turned-down, there is an increasing likelihood of liquid passing through the gas passageways thereby causing a reduction in tray stability. Accordingly, the tray is designed to have a high enough dry plate pressure drop at lower gas flows to ensure adequate tray stability at the lower gas flow rates for anticipated turn-down conditions.
One skilled in the art will readily appreciate that in order to ensure adequate stability at turn-down, this design procedure imposes a substantial pressure drop penalty at non turn-down design conditions. This penalty becomes especially severe in vacuum separation applications where a low pressure drop is extremely advantageous to avoid both excessive column diameters due to low overhead pressures and potential product degradation due to high base temperatures.
Another prior art approach to maintaining tray stability at turn-down has been to over-reflux the column so as to maintain the proper gas and liquid traffic within the column. However, this approach considerably increases the energy requirements per unit of product of the distillation system. Such increased energy requirements are undesirable due to high energy costs.
Yet another prior art approach to maintain tray stability at turn-down has been to blank off or obstruct a portion of the gas passageways of the gas-liquid contacting member of a sieve tray so as to reduce the flow area available for gas flow. This permits the dry plate pressure drop to be maintained at a satisfactory value to ensure adequate tray stability at the lower volumetric gas flow rates present when the column is operating in a turned-down condition. However, this is essentially a stop-gap approach which serves to increase costs since implementation requires extensive equipment modifications. Moreover, this approach is not a practical way to deal with daily variations.